A High Stability Clock Generator
In contrast, a high stability, temperature compensated quartz clock generator will show approximately +/- 0.5 ppm initial frequency uncertainty, a temperature dependent drift of +/- 1 ppm over the operating temperature range (again 0...50°C) and ageing of less than 1 ppm / year. The same alarm clock now build with this clock generator would give the time correctly to better than a minute already. If the initial frequency uncertainty and ageing of the quartz crystal could be compensated for to, say 0.25 ppm residual uncertainty, the alarm clock would even be precise to 8 seconds per year worst case.
For the ALV-7002/USB correlator such a high stability clock generator can be ordered as option, for all other ALV correlator products this option will become available soon. The high stability clock generator offers the following performance:
A special software package comes along with the above outlined clock generator option. Using this and feeding the PPS (pulse per second) output of a GPS receiver into the correlator input the exact frequency of the clock generator can be determined and ageing effects can be compensated via a trimmer accessible from the below side of the correlator housing.
Typically commercial grade GPS receivers with PPS output ensure a PPS stability of better than 1 µs (1 ppm), special GPS receivers designed for timing applications ensure a PPS stability of better 25 ns (0.025 ppm). While in the first case, measurement times of a few 10 seconds must be expected to average the (random) uncertainty in the PPS pulse, the use of a high precision GPS receiver allows practically “real-time” measurements of the actual clock generator frequency with updates every second. Alternatively, a stable high frequency clock (a frequency standard, for example) with either 5 or 10 MHz frequency can be fed to the correlator as well. If so, a frequency stability for the frequency standard of better 0.1 ppm is required.
Using this special software package and a GPS receiver with PPS output (or a frequency standard), the user can easily re-trim the stable clock generator to better 0.25 ppm absolute on-site within just a few minutes.
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